1. Field of the Invention
The present invention relates in general to the wireless communication field and, in particular, to a transmitter that can exploit feedback about a state of a channel to help it organize information which is subsequently transmitted to a receiver.
2. Description of Related Art
The use of coding, interleaving and modulation is prevalent in both current and proposed wireless communication systems. Also, the current and proposed wireless communication systems can have different forms of radio access including Time Division Multiple Access (TDMA) with or without frequency hopping, Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiplexing (OFDM), Multi-Carrier CDMA (MC-CDMA), etc. In addition, the current and proposed wireless communication systems enable many different forms of feedback signaling, ranging from power control commands to complete channel state information. These main ingredients can be combined in different ways within the current and proposed wireless communication systems. To help describe how these main ingredients can be combined within traditional wireless communication systems, reference is made to FIG. 1.
FIG. 1 (PRIOR ART) is a block diagram showing the basic components of a traditional wireless communication system 100 that includes a single-antenna transmitter 102 (only one shown) and a single-antenna receiver 104 (only one shown). One skilled in the art will appreciate that it is relatively straightforward to extend this diagram and the description below to take into account multiple-antenna transmitters and/or multiple-antenna receivers. As shown, the transmitter 102 has a coder 106, an interleaver 108 and a modulator 110 that work together to transmit a radio signal 112 which passes through a channel and is received by the receiver 104. The received radio signal 112 is made up of many components, including the desired signal, own cell interference, other cell interference, adjacent carrier interference, and internal receiver noise. In baseband, the received radio signal 112 can be written as:rn=C*sn+in+wn,where sn refers to the desired signal information symbols, C=[C0, . . . , CM-1] is a vector of M channel taps representing the state of the channel, * indicates a filter, or convolution, operation, in refers to one or more co-channel and adjacent channel interference signals that the receiver 104 treats explicitly (for instance by whitening or joint detection), and wn refers to all other noise.
The format of the desired signal sn may be TDMA, CDMA, OFDM, MC-CDMA, etc. . . . First consider the case of TDMA. On each slot, assume that the receiver 104 computes an estimate of the signal-to-noise-ratio (SNR). The SNR is presumed to account for the particular capabilities of the receiver 104. That is, if the receiver 104 suppresses co-channel or adjacent channel interference, then that is reflected in the SNR. In a non-frequency hopped TDMA system, the slot SNR's reflect the time evolution of the desired signal channel, interference, etc. . . . And, in a frequency hopped TDMA system, the SNR's reflect the frequency selectivity of the channel, and the different interference level on each hop. Next consider the case of OFDM and MC-CDMA, where it is assumed that the receiver 104 computes an SNR for each carrier. These SNRs reflect the frequency selectivity of the channel, and the different interference level on each carrier. In all of these cases associated with TDMA, CDMA, OFDM, MC-CDMA, etc. . . . , the receiver 104 sends the SNR information in a feedback signal 114 to the transmitter 102.
Unfortunately, the traditional transmitter 102 does not use the SNR information in the feedback signal 114 to help organize the information it subsequently transmits in a radio signal 116 to the receiver 104. Instead, the transmitter 102 uses a randomizing strategy or some other strategy to send the information in radio signal 116 to the receiver 104. A more detailed discussion is provided next about some of the different ways the transmitter 102 can use a randomizing strategy or some other strategy to send the information in radio signal 116 to the receiver 104.
First, assume the transmitter 102 protects the information by using a random error correcting code. The random error correcting code can include convolutional codes, turbo codes, binary block codes or low density parity check codes (for example). Also, assume that the transmitter 102 uses trellis coded and block coded modulation schemes. These coding and modulation schemes are designed, and work best for the transmitter 102 in a non-fading environment where the presence of a noise process is independent from bit to bit (or symbol to symbol). However, in a fading environment for these methods to work well, the transmitter 102 typically resorts to a randomizing strategy which uses some form of interleaving to try and re-create a favorable scenario that takes advantage of diversity to transmit the information in radio signal 116 to the receiver 104. The term diversity is used herein to cover the variation in the desired signal's channel conditions, as well as the level of interference and its channel conditions, which are all seen from the vantage point of the receiver 104.
For instance, in a non-frequency hopping TDMA system, the transmitter 102 interleaves the bits from a codeword over multiple slots and within each slot to benefit from time diversity. In a frequency hopping TDMA system, the transmitter 102 interleaves the bits from a codeword over multiple hops, to benefit from frequency diversity. This is typically done in addition to time diversity. For a more detailed discussion about the relationship between coding and hopping reference is made to U.S. Patent Application Serial No. 2002/0126736 entitled “Methods and Systems for Selective Frequency Hopping in Multiple Mode Communication Systems”.
Similarly, in an OFDM or MC-CDMA system, the transmitter 102 interleaves the bits from a codeword over multiple carriers. Thus, the guiding principle of a randomizing strategy that uses interleaving is to subject each codeword to a diversity of channel conditions, some of which are favorable, to give the receiver 104 a good chance of decoding that codeword successfully.
Now assume the transmitter 102 protects the information by using a burst error correcting code. The burst error correcting code includes binary codes such as Fire codes, and non-binary codes such as Reed-Solomon codes. In this situation, the strategy is essentially the opposite of randomizing. That is, for the case of a binary code, the transmitter 102 places neighboring bits on the same modulation symbol, and on the same slot or on the same tone. That way, if conditions at the receiver 104 are bad on a certain modulation symbol or slot or tone, a burst error may occur, which a decoder located therein is well suited to handle. Similarly, for the case of a non-binary code, the bits representing a non-binary code symbol (which may or may be the same size as a modulation symbol) are placed on the same modulation symbol, and on the same slot or on the same tone. As can be seen, the transmitter 102 and the burst error correcting code scheme described in this example do not use the SNR information in the feedback signal 114 to help organize the information it subsequently transmits in radio signal 116 to the receiver 104.
Now assume the transmitter 102 protects the information by using error control coding which is found in differential modulation and coding schemes. Typically, differential modulation is used for reasons other than coding. But, in here differential modulation is viewed from the coding perspective. For instance, consider the IS-136 standard, which uses Differential Quadrature Phase Shift Keying (DQPSK). In this case, the transmitter 102 while in speech mode protects certain information bits by coding while other information bits remain uncoded.
It has been found that the performance of DQPSK can be enhanced with the use of multi-pass demodulation. This enhancement performance is described in U.S. Pat. No. 5,673,291. Basically, the patent describes an idea where re-encoded DQPSK symbols corresponding to protected bits are used as effective pilots, to help demodulate the received neighboring symbols.
The multi-pass demodulation scheme itself can also be improved upon as described in an article by A. Khayrallah et al. entitled “Interleaver Design and Multi-Pass Demodulation,” Proceedings Conference on Information Sciences and Systems, 2001. Using the scheme described in this article, one can provide large performance gains when the original interleaver is replaced with one designed specifically to take advantage of the interplay between multi-pass demodulation and the differential properties of DQPSK.
It has also been found where it is useful to separate the differential aspect from the modulation itself. That is, one can use standard coherent modulation such as Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) and then impose differential relations among the bits before mapping them into modulation symbols. An advantage of this is associated with the added flexibility one has to design various differential relations among the bits. Moreover, it has been shown in the following two documents that such differential schemes can provide large gains in conjunction with multi-pass demodulation. One document is U.S. Patent Application No. 2001/0033621 and the other document is an article by A. Khayrallah entitled “Differential Coding over Bits for Higher Level Modulation,” Proceedings Conference on Information Sciences and Systems, 2002. The contents of both of these documents are incorporated by reference herein. In these schemes, the basic idea is that re-encoded bits can act as effective pilots, boosting the performance of neighboring bits that are connected to them by a differential relation. As can be seen, the transmitter 102 and the differential modulation and coding schemes described in this example do not use the SNR information in the feedback signal 114 to help organize the information it subsequently transmits in radio signal 116 to the receiver 104.
Lei et al. “Performance Analysis of Adaptive Interleaving for OFDM Systems”, IEEE Transactions on Vehicular Technology, Vol. 51, No. 3, May 2003 is an article that discloses a transmitter which uses an adaptive interleaving technique that rearranges symbols according to instantaneous channel state information (CSI) of OFDM subcarriers so as to reduce or minimize the bit error rate (BER) of each OFDM frame transmitted to a receiver. The receiver estimates the CSI for transmitted frames and then feedbacks the CSI to the transmitter. The transmitter uses the CSI to predict future CSIs and then determines the most efficient adaptive interleaving pattern. The receiver would estimate the same future CSIs and determine an identical interleaving pattern for de-interleaving.
US 2002/016938 A1 (Thomas J. Starr et al.) Feb. 7, 2002 discloses a method and system for automatically controlling an adaptive interleaver which involves monitoring performance parameters (SNR, BER . . . ) of a transmission system and then controlling the adaptive interleaver in response to the monitored performance parameters. In one embodiment, there is a transmission system 180 which comprises an adaptive interleaver 20, a transmitter 30, a transmission channel 35, a receiver/decoder 40 and a controller 80. The controller 80 comprises a signal to noise ratio monitor 72 which monitors the SNR on transmission channel 35 and generates a multiple bit adaptive interleave control signal 74 that preferably varies as a function of the SNR. Alternatively, the adaptive interleave control signal 74 can be binary such that the adaptive interleave control signal produced is greater than or less than a threshold value based upon the SNR. The controller 80 is coupled with to the adaptive interleaver 20 such that the adaptive interleave control signal 74 is supplied directly to the adaptive interleaver 20. The adaptive interleave control signal 74 is preferably utilized by the adaptive interleaver 20 to control the interleave depth to generate an adaptively interleaved signal.
Fengye Hu et al. “An Adaptive Interleaving Scheme for MIMO-OFDM Systems”, Emerging Technologies: Frontiers of Mobile and Wireless Communication, May 31-Jun. 2, 2004 is an article that discloses an adaptive interleaving scheme for MIMO-OFDM systems. To perform this adaptive interleaving scheme, the receiver needs to estimate the channel state information (CSI) of received OFDM signals. Then, the receiver and transmitter need to use an identical predictive filter and have the same copy of CSI sequence so they both can use an identical interleaving/de-interleaving pattern.
US 2002/176482 A1 (Charles Chien et al.) Nov. 28, 2002 discloses a radio transceiver module (including a control processor) interfaced with a channel monitor which monitors a communication channel and estimates its characteristics from time to time, thus providing a dynamic estimate of channel characteristics. Based on the channel characteristics, the control processor calculates a preferred configuration of digital (and optionally, analog) signal processing to best manage the available energy for the present channel characteristics. The selected configuration is then down-loaded into communication modules stored in extra memory during runtime. The communication modules preferably include a one or more of: a reconfigurable forward error correcting codec (with adjustable code lengths and a plurality of code choices); a reconfigurable interleaver with adjustable depth; a decision feedback equalizer (DFE) with a reconfigurable number of taps; a maximum likelihood sequence estimator with an adjustable number of states; a frequency hopping coder with an adjustable number of hops or hop rate; and a direct-sequence (or direct sequence spread spectrum) codec with an adjustable number of chips per bit.
From the foregoing, it should be noticed that there is a need for a transmitter to use information in a feedback signal to help configure/select a randomizing strategy or some other strategy like differential modulation and coding to organize and send information in radio signal to receiver. The exploitation of information within a feedback signal, to help a transmitter better organize the information that is transmitted in a radio signal to a receiver is addressed by the present invention.